(DiamondLightSource/dodal#751) Draw crosshairs with better contrast

src/dodal/devices/areadetector/plugins/MJPG.py

@@ -91,7 +91,8 @@ class SnapshotWithBeamCentre(MJPG):
     image and saves the image to disk."""
 
     CROSSHAIR_LENGTH_PX = 20
-    CROSSHAIR_COLOUR = "Blue"
+    CROSSHAIR_OUTLINE_COLOUR = "Black"
+    CROSSHAIR_FILL_COLOUR = "White"
 
     def post_processing(self, image: Image.Image):
         assert (
@@ -100,15 +101,27 @@ def post_processing(self, image: Image.Image):
         beam_x = self.oav_params.beam_centre_i
         beam_y = self.oav_params.beam_centre_j
 
+        SnapshotWithBeamCentre.draw_crosshair(image, beam_x, beam_y)
+
+        self._save_image(image)
+
+    @classmethod
+    def draw_crosshair(cls, image: Image.Image, beam_x: int, beam_y: int):
         draw = ImageDraw.Draw(image)
-        HALF_LEN = self.CROSSHAIR_LENGTH_PX / 2
+        HALF_LEN = cls.CROSSHAIR_LENGTH_PX / 2
+        draw.rectangle(
+            [beam_x - 1, beam_y - HALF_LEN - 1, beam_x + 1, beam_y + HALF_LEN + 1],
+            fill=cls.CROSSHAIR_OUTLINE_COLOUR,
+        )
+        draw.rectangle(
+            [beam_x - HALF_LEN - 1, beam_y - 1, beam_x + HALF_LEN + 1, beam_y + 1],
+            fill=cls.CROSSHAIR_OUTLINE_COLOUR,
+        )
         draw.line(
             ((beam_x, beam_y - HALF_LEN), (beam_x, beam_y + HALF_LEN)),
-            fill=self.CROSSHAIR_COLOUR,
+            fill=cls.CROSSHAIR_FILL_COLOUR,
         )
         draw.line(
             ((beam_x - HALF_LEN, beam_y), (beam_x + HALF_LEN, beam_y)),
-            fill=self.CROSSHAIR_COLOUR,
+            fill=cls.CROSSHAIR_FILL_COLOUR,
         )
-
-        self._save_image(image)

tests/devices/unit_tests/areadetector/plugins/test_MJPG.py

@@ -1,6 +1,8 @@
+from pathlib import Path
 from unittest.mock import MagicMock, patch
 
 from ophyd.sim import instantiate_fake_device
+from PIL import Image
 
 from dodal.devices.areadetector.plugins.MJPG import SnapshotWithBeamCentre
 from dodal.devices.oav.oav_detector import OAVConfigParams
@@ -31,3 +33,13 @@ def test_given_snapshot_triggered_then_crosshair_drawn(
     status.wait()
 
     assert len(patch_line.mock_calls) == 2
+
+
+def test_snapshot_draws_expected_crosshair(tmp_path: Path):
+    image = Image.open("tests/test_data/test_images/oav_snapshot_test.png")
+    SnapshotWithBeamCentre.draw_crosshair(image, 510, 380)
+    image.save(tmp_path / "output_image.png")
+    expected_image = Image.open("tests/test_data/test_images/oav_snapshot_expected.png")
+    image_bytes = image.tobytes()
+    expected_bytes = expected_image.tobytes()
+    assert image_bytes == expected_bytes, "Actual and expected images differ"